src/mkhelp: strip off escape sequences

At some point the nroff command stopped stripping off escape sequences,
so then this script needs to do the job instead.

Reported-by: VictorVG on github
Fixes #11501

src/mkhelp.pl

@@ -51,6 +51,7 @@
     # remove trailing CR from line. msysgit checks out files as line+CRLF
     $line =~ s/\r$//;
 
+    $line =~ s/\x1b\x5b[0-9]+m//g; # escape sequence
     if($line =~ /^([ \t]*\n|curl)/i) {
         # cut off headers and empty lines
         $wline++; # count number of cut off lines